Ventricular tachycardia associated with desipramine and thioridazine

Systematic overview of drug interactions with antidepressant medications

OBJECTIVE

Antidepressants are commonly used drugs with potential for numerous drug interactions. This study aims to systematically review the literature on drug interactions with antidepressants.

METHODS

We searched MEDLINE (1966 to November 2003) and EMBASE (1980 to 2003), using the heading drug interactions combined with individual antidepressant names. We restricted searches to English-language articles and human studies. We screened drug interaction texts and review articles for relevant studies. We included articles reporting original human data on drug interactions with antidepressants commonly used in North America. Articles were independently evaluated by 2 reviewers on clinical effect, clinical significance, and quality of evidence. Discrepancies were resolved by consensus.

RESULTS

There were 904 eligible interactions, involving 9509 patients, for a total of 598 summary interactions. Of these, 439 (73%) demonstrated an interaction, 148 (25%) had no effect, and 11 (2%) had conflicting evidence. For 510 interactions (85%), the quality of evidence was poor. It was fair for 67 (11%) interactions and good for 10 (2%) interactions. There were no interactions with excellent quality of evidence. There were 145 (24%) interactions of major clinical significance. These were predominantly hypertensive emergencies and serotonin syndrome. Most interacting drugs had central nervous system (CNS) activity. As expected, monoamine oxidase inhibitors (MAOIs) appear to be the most problematic family in terms of potential for serious drug interactions.

CONCLUSIONS

Drug interactions with antidepressants are an important cause for concern, but this concern is based primarily on poor evidence. We recommend caution when combining antidepressants with other CNS drugs, particularly when coadministering MAOIs with other substances.

Pharmacokinetic Factors in the Adverse Cardiovascular Effects of Antipsychotic Drugs

Antipsychotics may cause serious adverse cardiovascular effects, including prolonged QT interval and sudden death. This review considers antipsychotic-induced cardiovascular events from three perspectives: high-risk drugs, high-risk individuals and high-risk drug interactions. Pharmacokinetic drug interactions involving the cytochrome P450 (CYP) enzymatic pathway and pharmacodynamic interactions leading to direct cardiotoxic effects are discussed. Original reports on antipsychotic-induced drug interactions are reviewed, with consideration of management guidelines. The literature was reviewed from 1 January 1966 to 1 February 2002. The literature search revealed only 12 original articles published on antipsychotic drug interactions leading to cardiovascular adverse events. Only 4 of the 12 reports were prospective studies; the remainder were either retrospective or anecdotal.Although poor study designs preclude a definitive statement, it appears that pharmacokinetic interactions primarily involved the CYP2D6 and CYP3A4 enzymatic pathways. Those involving the CYP2D6 isozyme included interactions with tricyclic antidepressants, selective serotonergic reuptake inhibitors and beta-blockers. Among these drug interactions, tricyclic antidepressants were most likely to reach clinical significance because of their limited therapeutic index. Drug interactions related to the CYP3A4 pathway were generally less severe, and involved high-potency antipsychotics coadministered with inhibitors such as clarithromycin. Strategies are discussed for the management of adverse cardiovascular events related to antipsychotic drug interactions, including the use of an algorithm. Large, randomised, placebo-controlled studies with strict inclusion criteria are needed to determine the role that antipsychotics play in QT prolongation and sudden death.

Effect of thioridazine dosage on the debrisoquine hydroxylation phenotype in psychiatric patients with different CYP2D6 genotypes

Sixteen hospitalized white European Spanish psychiatric patients treated with thioridazine alone were studied with respect to CYP2D6 genotype, debrisoquine metabolic ratio (MR), and the plasma levels of thioridazine and its metabolites mesoridazine and sulforidazine. After decreasing the dose of thioridazine the debrisoquine MR and thioridazine plasma levels were redetermined. At the initial determination (regular clinical doses, 20-300 mg/day), 14 of 16 patients (88%) were classified as poor metabolizers of debrisoquine (PMs). However, after complete withdrawal of thioridazine in 10 patients, all 10 became extensive metabolizers except two who were genotypically PMs (*4/*4). The inhibition of debrisoquine metabolism was genotype dependent. All patients with wt/wt genotype treated with a dose 150 mg/d were phenotypically PMs, all patients with wt/*4 genotype treated with a dose of 50 mg/d or greater were PMs. The debrisoquine MR from all dose changes correlated with the dose (p < 0.001) and plasma level (p < 0.001) of thioridazine. The CYP2D6 hydroxylation capacity was inhibited by thioridazine as determined by the debrisoquine MR. This inhibition was reversible by thioridazine withdrawal, and thus seems to be dose dependent and related to CYP2D6 genotype. One must consider the effects of thioridazine dosage on CYP2D6, because it may influence the metabolism of concomitant drugs or produce clinically important adverse effects such as cardiotoxicity. An awareness of this problem and cautious dosage adjustment of other drugs metabolized by the same enzyme are recommended during treatment with thioridazine.

Use of the mesoridazine/thioridazine ratio as a marker for CYP2D6 enzyme activity

Thioridazine is metabolized in humans by CYP2D6 to mesoridazine, which is an active metabolite. Two or more CYP2D6 substrates are seldom given simultaneously to elderly patients because potentially dangerous metabolic interactions may occur. It may be valuable to know the CYP2D6 metabolic capacity of such patients to avoid drug interactions, which depend on the metabolic phenotype. The goal of this study was to evaluate the use of the mesoridazine/thioridazine ratio for the estimation of CYP2D6 enzyme capacity. A sensitive and reliable method has been developed for the determination of thioridazine and its metabolites, mesoridazine and sulforidazine. Commonly used central nervous system (CNS) comedications do not interfere with the method. A group of 27 chronic patients with mental illness receiving monotherapy with thioridazine were studied. There were 23 men and 4 women between 37 and 80 years old (mean +/- SD: 61.2 +/- 10.2). The thioridazine/mesoridazine ratio correlated with the debrisoquine metabolic ratio (r = 0.74, p < 0.001). Therefore, the authors suggest that the measurement of thioridazine and its metabolite might be a useful tool to assess CYP2D6 activity during treatment.

Clinically significant interactions of psychotropic agents with antipsychotic drugs

Various psychotropic drugs are commonly combined with antipsychotic agents. Such combinations can induce pharmacodynamically based, presumably additive, beneficial (e.g. sedative or mood-altering) effects or adverse autonomic, cardiac depressant and CNS intoxicating effects. Clinically significant interactions also arise through competition with or induction of hepatic microsomal cytochrome P450 (CYP) enzymes, particularly the CYP1A2 and CYP2D6 isozymes by which most antipsychotics are oxidised. Such pharmacokinetic interactions can elevate circulating concentrations of antipsychotics (both typical agents and the atypical antipsychotic clozapine) to potentially toxic ranges, which may lead to increased risks of adverse effects. Such interactions occur particularly with serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitor antidepressants. Metabolic interactions that lead to lesser increases in antipsychotic concentrations may arise in combining these drugs with other antidepressants, benzodiazepines or propranolol. In contrast, most anticonvulsants, except valproic acid (sodium valproate), induce the oxidative metabolism of antipsychotics and can lower their plasma concentrations to potentially subtherapeutic levels, with unpredictable increases after their discontinuation. Since simultaneous use of multiple psychotropic agents is increasingly common, special caution is required to avoid untoward consequences of interactive adverse effects due to drug interactions, which can sometimes be severe or life-threatening.